Oct., 1923] ROBBINS — ISOELECTRIC POINT FOR PLANT TISSUE 429 
sodium citrate-NaOH series the minimum was found at pH 5.5-57; and 
with the potassium hydrogen phthalate-NaOH series the minimum was 
located at a pH of 6.2-6.25. In all three sets of the H 3 P 0 4 -Na 0 H series, 
the least change in weight occurred in the solutions having a pH of 5.4. 
This was also true in one experiment with the secondary sodium citrate- 
NaOH buffer mixtures. The shapes of the curves were in each case much 
the same for all three weighings at 8, 12, and 24 hours, 12, 24, and 48 hours, 
or 6, 12, and 24 hours. The shape of the curve and the location of the 
minimum point for the turgid tissue at the end of 6 hours was the same as 
for the flaccid and presumably dead tissue at the end of 24 hours in the 
citrate and phthalate series. 
The differences in weight between the potato in the solutions where the 
minimum weight was found and that of the potato in the solutions producing 
the maximum weight on the acid or alkaline sides of the minimum were not 
great. As shown in table 5, they ranged from 0.6 percent to 8.5 percent of 
the maximum total weight. This difference appears insignificant. Since, 
however, a large percentage of water must be present in resting tissue, it 
would appear that the difference expressed as percentage of gain rather than 
percentage of total weight might be of greater physiological importance. 
It is the small additional amount of water absorbed that is responsible for 
increased turgidity and for the phenomena which accompany it. If we 
consider the differences in weights from this standpoint they become more 
significant. Expressed as percentage of maximum gain, the majority of 
the differences were in the neighborhood of 25 percent. In other words, the 
gain of the potato tissue in solutions showing the minimum point was about 
25 percent less than it was where the maximum gain was found on either side. 
When the amount of the differences in gain in weight for the potato at 
the minimum and at the maximum on either side for the different buffer 
mixtures is considered, it would appear that in the phosphate series the 
more dilute the buffer mixtures of a given set the greater the difference. 
Thus, at the end of 12 hours the differences for the 0.1 M series and the 
two 0.01 M series between the acid maximum and the minimum were 
0.29 g., as compared to 0.59 or 0.67 g. and 0.73 g. respectively. For the 
alkaline maximum and the minimum the difference was 0.32 g. compared to 
0.64 g. and 0.41 g. With the citrate series the situation is complicated by 
the greater toxicity of the citrate ion. 
Attention should also be called to the fact that it was only with the 
0.002 M H3PO4-0.002 NaOH sets of buffer mixtures that gains as great as 
those found in distilled or redistilled water were observed. 
A consideration of the change in reaction which the buffer mixtures 
showed as a result of contact with the potato indicates in general that up to 
pH 5.5-5.8 the solutions became more alkaline, while in solutions more 
alkaline than pH 5.5 or 6.1 they remained unchanged or became more acid. 
The results of the changes which occurred in the reaction of the buffer 
mixtures due to the presence of the potato are summarized in table 6. 
32 
